1. Field of the Invention
The present invention is broadly related to a generator and a corresponding method for fumigating a commodity with a phosphine gas. Phosphine gas is generated by mixing a metal phosphide and water upon agitation and dilution with air.
2. Description of Related Art
Fumigation of stored agricultural commodities such as grains with phosphine gas is the foremost method of preventing insect damage. Typically, fumigation is achieved by introducing pellets or tablets containing metal phosphide directly into the grain to be fumigated. The metal phosphide reacts with the ambient moisture in the air and grain, resulting in the generation of a phosphine gas and other inert gases. Forced air circulation devices are often used to assist in distribution of the phosphine gas throughout a storage structure, such as a grain silo.
A common fumigation problem encountered with prior practices is the inability to achieve a uniform concentration of phosphine gas within the storage structure quickly. It is known that for the most effective insect control, it is necessary to maintain the desired concentrations of phosphine for known periods. However, with prior methods the release of phosphine gas is slow and takes three to seven days or more depending on the ambient conditions.
Batch processes for the generation of phosphine gas have been proposed in the past. Such batch processes include a batch reactor for the hydrolysis of metal phosphides to obtain phosphine gas, the latter being stored in closed cylinders. Such containers can then be used on-site to deliver phosphine gas at a specific concentration throughout a selected period of fumigation. A significant drawback of this technique is that one must store the phosphine gas in pressure vessels with subsequent delivery of the vessels to the storage structure. This results in a need for expensive cylinders and poses handling hazards. Also, the phosphine gas must be transported to the fumigation site.
A very common fumigation problem encountered with prior practices is the use of CO2, N2, argon, and other similar gases which aim to dilute the phosphine gas concentration and to maintain low levels of phosphine gas until the phosphine gas reaches the storage structure for fumigation. At the same time, prior practices require an increased inventory and additional handling and hazards of pressurized cylinders.
Prior methods and devices for generating phosphine gas using metal phosphide have a number of additional problems. First, any unreacted metal phosphide in the reaction mixture would remain in the commodity after fumigation. Therefore, the metal phosphide had to be withdrawn in the form of a fine powder or collected in a bag. This handling of the unreacted phosphide metal poses increased health concerns to the operator. Second, the temperature of the reaction pot would increase during the reaction, since the reaction is exothermic. Hence, cooling jackets were needed around the reaction pot which increases the cost of generating the phosphine gas.
There is accordingly a need in the art for an improved apparatus and method allowing the safe, rapid production of phosphine gas for on-site generation. The desired apparatus and method would produce the desired phosphine gas concentration for fumigation while avoiding the need to produce phosphine gas at a remote site using intermediate containers of phosphine gas. Also, an apparatus that generated phosphine gas on a continuous basis without the addition of rotating parts like agitators, stirrers and rotors would be advantageous. Additionally, any improved apparatus or method would reduce or eliminate any hazards associated with the prior methods, including eliminated the usage of inert gases.